1. Field Of The Invention
The present invention relates to a porous membrane comprising a polyvinylidene fluoride resin and having a uniform, three-dimensional, network pore structure and excellent chemical resistance, excellent filtering characteristics and excellent mechanical properties. The present invention is also directed to a method for producing the porous membrane.
2. Discussion Of Related Art
It is known that polyvinylidene fluoride resin is a useful material for porous membranes having excellent chemical resistance, thermal resistance and mechanical properties.
Heretofore, various porous membranes comprising a polyvinylidene fluoride resin have been proposed. Most of the conventional porous membranes are produced by the wet methods. These conventional membranes have non-uniform porous structures and also have skin layers and, therefore, are poor in mechanical strength. In Japanese Patent Application Laid-Open Specification No. 60-97001, one of the wet methods for preparing a porous membrane having a network structure is proposed. However, the proposed wet method involves a problem in that the membrane produced by the method is poor in mechanical strength. On the other hand, in Japanese Patent Application Laid-Open Specification No. 58-93734, a method is proposed for producing a porous membrane, which comprises blending a polyvinylidene fluoride resin, an organic liquid and a hydrophilic inorganic filler, subjecting the resultant blend to melt-molding to form a membrane and extracting the organic liquid and the hydrophilic inorganic filler from the membrane. However, the porous membrane produced by this method has a disadvantage in that it has many macrovoids and is poor in tensile elongation at break. Therefore, the porous membrane cannot be used at high temperature under high pressure.
As described above, heretofore, a porous membrane having a uniform, three-dimensional, network pore structure and having excellent mechanical strength has not been produced.